HAREM, Syria (Reuters) - Crouching in a tent among the pine trees, two rain-soaked men trace a map in the dirt. A cigarette stub, a rock and a tuna can mark targets amid a scatter of X-marks and arrows.

They might almost be football coaches making a gameplan, but in Syria these men are making war.

Fighters run in from the deluge, yelling for ammunition and transport for the wounded. Thunder rumbles in the distance and blends into bursts of mortar fire.

Shouting over the din, the two commanders debate tactics for taking the town of Harem, which their rebel forces have under siege. How they fare has big implications, not just because the ancient strongpoint on the Turkish border dominates a strategic route to Aleppo, but as a test of opposition efforts to better marshal their untrained and fractious bands of volunteers.

"Basel, listen!" shouted Abu Osama, one of the two leaders at the rain-sodden command post, to fellow rebel commander, Basel Eissa, as they hammer out a coordinated plan for their brigades. "This revolution has been disorganized and random for over a year now. It's time to start focusing our strategies.

"All I hear from the fighters is 'Storm the city! Storm the city!' - before we've secured any territory. I'm sick of this slogan. Hold them back until our units have bombed the targets."

That was nearly two weeks ago, when a Reuters news team began observing the siege of Harem, which began in mid-October. This week, troops loyal to President Bashar al-Assad are holding out in the mediaeval citadel after the rebels drove them from the rest of the town, fighting house to house, street by street.

Sniper fire and air strikes continue to take a toll on the 500 or so besieging rebels who, in reply, offered little quarter to some prisoners - one of whom Reuters saw them shoot dead.

As Assad's opponents and their Western, Arab and Turkish backers meet in Qatar this week seeking elusive unity, Harem shows rebel commanders struggling to forge a single, disciplined force which might ease foreign powers' fear that arms sent to shadowy groups may simply fuel carnage - or even be turned against their donors.

Whether the opposition can succeed, remains unclear. Its political leadership has appeared as divided as ever in Doha, frustrating the hopes of allies like U.S. Secretary of State Hillary Clinton, who is pushing for a united approach that gives a strong voice to those "in the front lines fighting and dying".

The fighting at Harem has shown rebels under the guidance of defectors from Assad's army like Abu Osama out-maneuvering better armed troops. But it also demonstrates the firepower, notably in the air, which Assad is deploying to defend his rule as the 19-month-old conflict, now all-out civil war, grinds on.

COMMANDING AMBITION

Abu Osama, a muscular artillery major who stands tall above his comrades in his tan army boots, joined the battle for Harem as a representative of the Joint Leadership of the Military Councils - a body whose own ultimate command structure is opaque but which says it aims to use funding and weaponry apparently mainly from Gulf states to take charge of the overall rebellion.

Imposing that leadership is not easy. The Councils have met skepticism from some rebels and are flatly rejected by others.

The bands of fighters, recruited from villages or city blocks, or by small political or religious groupings, have been left to their own devices so far; many suspect senior army defectors of corruption and trying to grab power for themselves.

But Harem, whose stone houses clustered under the fortress and surrounded by pomegranate groves are home to about 20,000 people, has provided the Councils and men like Abu Osama with a chance to show them what tactical leadership can provide.

In past months, attempts to rush the defenses of a town once garrisoned by Crusader knights did little but add dozens more rebel lives to the tens of thousands lost in a conflict that began in last year's Arab Spring street protests.

Now, with a new plan supervised by trained army officers from the Joint Military Councils, a tightly organized siege is in place. Rebels say new tactics have cut their casualty rate even as Assad's men are fighting for their lives in the castle.

At the rebel command post on a muddy hillside, Abu Osama, the 39-year-old officer from southern Syria, tried to reassure Eissa, 43, an auto parts salesman from the nearby city of Idlib now leading the local fighters of the Idlib Martyrs Brigade:

"You are right that we will not advance quickly, it will be slower. But it will hold," Abu Osama told a frowning Eissa, a father of four with no previous military experience. "We advance by firepower, not manpower. Let's preserve our men's lives."

BATTLE JOINED

Below them, however, in the winding alleys of the town, for the men trying to advance along the neat arrows of Abu Osama's map in the dirt, frustration and death were a constant presence.

Pushing towards the citadel, the base for an isolated force of some 400 loyalist soldiers and pro-Assad "shabbiha" militiamen, rebel fighters in a motley collection of camouflage fired their rifles - Kalashnikovs and M-16s - from the cover of street corners; rocket-propelled grenades crashed nearby.

Pinned back by a lethal curtain of sniper fire, rebels used sledgehammers to smash through internal walls between houses to advance under cover from room to room.

Leading from the front, Eissa, a burly man in a floppy black cowboy hat, was constantly wiping concrete dust from his curly beard. His voice is hoarse from shouting orders: "What are you doing?" he barked at two men idling at the back. "We're raiding a new position - join your unit at the front now."

Back up the hill, watching the battle unfold, was Mohammed al-Ali, previously a major in an army engineering unit. He said taking Harem would almost complete the opposition hold on Idlib province, creating a big, anti-Assad bridgehead between Aleppo, Syria's largest city, and supplies coming in from Turkey.

But perhaps more importantly, success here could, he hoped, demonstrate the kind of centralized command structure that would win the confidence of Western and Arab leaders skeptical of the rebels' cohesiveness and fearful they harbor in their ranks Islamist militants inspired by al Qaeda and hostile to the West.

"The Joint Military Councils have begun trying to organize forces under a shared plan, and Harem is the model," said Ali, coordinator for a Council plan to have experienced officers like himself move around the various fronts to improve cooperation, training and specialization: "We will have artillery units and assault units, anti-aircraft units and raiding units," he said.

The Joint Councils have more than just expertise to rely on. They say they have money, too - enough to offer a small monthly stipend of $150 to fighters in units that accept their command.

A member of the Free Syrian Army talks on the radio during an operation in Harem town, Idlib Governorate in this October 26, 2012 file photo.Asmaa Waguih/Files

Funding for the Councils comes from various states which back the rebels, notably Qatar, Saudi Arabia and Turkey, a Syrian exile leader, Burhan Ghalioun, told Reuters in Doha.

In northern Syria, another officer working with the Councils, Bashar Saadeddine, said: "The hope is that a salary will offer an incentive to draw units under our oversight and finally get some order in the ranks."

Among initiatives is an embryonic system of military courts set up to counter the kind of war crimes which have risked giving the opposition as bad a name internationally as Assad.

But like the commanders watching from their hilltop, the reality in the streets of Harem is far removed from such ideals.

SUMMARY KILLING

Last week, as rebel forces closed in on the citadel and the defenders fought back with new-found ferocity, anger mounted in the opposition ranks after one comrade was shot clean through the head by a sniper and a grenade killed three others.

With many loyalists not in uniform, rebel fighters fire at any man in civilian clothes whom they suspect of being shabbiha - the irregular auxiliaries drawn mainly from Assad's minority Alawite community who have been blamed for numerous massacres.

Surrender is no guarantee of survival. Outside one house, lying in a flower bed, lay four bodies in Syrian army camouflage - all had been shot in the head. Nearby, a man in plain clothes, was bleeding heavily. "They've killed me," he howled.

Though Eissa ordered his men to treat his wounds, when the commander returned some minutes later the man had bled to death.

From another house, rebel fighters hauled a bearded man in a track suit top. Unarmed, he made to flee, but the men around him fired, almost in slow motion, one shot, two, three. Silent, he fell dead on the street. They later justified their action, saying documents on the body showed he was a loyalist officer.

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Rebel commanders, discomfited, said it was a rare incident: "We've taken casualties all morning," said Mohaned Eissa, an aide to his brother Basel. "This is war. Some mistakes will be made."

Other government soldiers captured in Harem seem to have been treated in line with the international principles the Military Councils have embraced. At Harem's town jail, where rebels are holding 200 men, there was no obvious ill treatment. Major al-Ali, the Military Councils' coordinator, said summary executions were rare, though "a few mistakes are inevitable".

Some of the men under his notional command, expressed a more cynical view, however, suggesting that the presence of Military Councils officers simply meant rebel fighters now took care to kill prisoners less publicly.

"The Joint Military Council is just a name. The fighters are the same as they always were," said one bearded young fighter named Majed, as he took a break from battle to rest on the stoop of a farmhouse. "Not much has changed - except now we're supposed to get rid of these criminals when no one is looking."

Accepting a pomegranate from the farmer whose house it was, Majed flashed another man's identity card in his direction:

Eyes widening, Majed shouted into his walkie-talkie and ran off: "Don't shoot the prisoners! Wait! You have an innocent!"

WEARY CIVILIANS

Earlier in the conflict, many Syrians tended to take pride in declaring their allegiance to one side or the other. But in Harem today, most locals can speak only of pain and a desire to see the conflict ended.

"Do I even remember which side I'm on?" asked one man, Abu Khaled, who sat on the floor of his living room with three teenage nieces, silently watching the rebels from a window. "I want Bashar to go so we can end this. This is a civil war; we are watching our brothers kill each other in the streets."

The women complained of the mess left by rebels who had camped in the room. But Abu Khaled silenced their criticism of the young men: "These are small losses," he said. "Look out the window. We are watching a whole generation disappear."

By the first days of November, most of Harem's streets were under the control of rebels who announced the "liberation" of the town and began distributing rice, oil, noodles and water to families trapped in their homes during the battle.

With its forces ensconced in the citadel above, Assad's army has turned in the past week or so to aerial bombing. While Turkey and its Arab and Western allies debate whether to intervene - a move strongly opposed by Russia, China and Assad's regional ally Iran - the Syrian air force has had a free hand.

During one raid last week, women cradled screaming babies under the olive trees outside their homes, as one jet dropped a bomb that seemed to shake the whole town. When a helicopter then buzzed overhead, one mother shouted in panic: "Is it above us?"

"We could be next."

Instead, though, the helicopter dropped white bundles of supplies, to Assad's men in the fort. She sobbed with relief.

Up the hill, rebel commanders ignored the crashing bombs and had turned again to their maps in the dirt, making more plans.

But as they hammered the fortress walls with rocket after rocket, sending smoke curling above the rooftops, Harem Castle towered over its attackers, seeming, at least for now, as durable as the Assad family's four decades in power.

On Monday, an air strike hit a rebel unit; among the dead was the brigade commander, Basel Eissa.